A large proportion (some estimates are as high as twenty-five percent) of the electricity generated in the United States each year goes to lighting. Accordingly, there is an ongoing need to provide lighting which is more energy-efficient. It is well known that incandescent light bulbs are very energy-inefficient light emitters—about ninety percent of the electricity they consume is released as heat rather than light. Fluorescent light bulbs are more efficient than incandescent light bulbs (by a factor of about 10) but are still less efficient than solid state light emitters, such as light emitting diodes.
In addition, as compared to the normal lifetimes of solid state light emitters, e.g., light emitting diodes, incandescent light bulbs have relatively short lifetimes, i.e., typically about 750-1000 hours. In comparison, light emitting diodes, for example, have typical lifetimes between 50,000 and 70,000 hours. Fluorescent bulbs have longer lifetimes (e.g., 10,000-20,000 hours) than incandescent lights, but provide less favorable color reproduction.
Another issue faced by conventional light fixtures is the need to periodically replace the lighting devices (e.g., light bulbs, etc.). Such issues are particularly pronounced where access is difficult (e.g., vaulted ceilings, bridges, high buildings, traffic tunnels) and/or where change-out costs are extremely high. The typical lifetime of conventional fixtures is about 20 years, corresponding to a light-producing device usage of at least about 44,000 hours (based on usage of 6 hours per day for 20 years). Light-producing device lifetime is typically much shorter, thus creating the need for periodic change-outs.
Accordingly, for these and other reasons, efforts have been ongoing to develop ways by which solid state light emitters can be used in place of incandescent lights, fluorescent lights and other light-generating devices in a wide variety of applications. In addition, where light emitting diodes (or other solid state light emitters) are already being used, efforts are ongoing to provide light emitting diodes (or other solid state light emitters) which are improved, e.g., with respect to energy efficiency, efficacy (lm/W), and/or duration of service.
With regard to embodiments in which the light emitter comprises one or more solid state light emitter, in many instances, a plurality of solid state light emitters are provided which are of different colors which, when mixed, are perceived as the desired color for the output light (e.g., white or near-white). The intensity of light emitted by solid state light emitters (e.g., light emitting diodes which in many instances further comprise one or more luminescent materials), when supplied with a given current, can vary (e.g., depending on the ambient temperature and/or the age of the solid state light emitter). Because of such potential variance, such lighting devices sometimes are provided with one or more sensors which detect (1) the color of the light being emitted, and/or (2) the intensity of the light being emitted from one or more of the solid state light emitters, and/or (3) the intensity of light of one or more specific hues of color, whereby the current supplied to different solid state light emitters can be adjusted as necessary in order to maintain the color of the output light within the desired range of color.
In addition, there exist a wide variety of other devices which include one or more light emitters and one or more sensors.